Susan, ask your son’s bass teacher for solos your son might play in a year or two. These will be sold with piano accompaniment, usually. Look at that accompaniment and simplify it for your use, even leaving some notes in the treble for the violinist. Now you have a trio, and though the first piece may be a bit clumsy, the more you do this the easier it gets. Then you could look for some easy violin solos in a collection her teacher suggests. Give her most of the melody but begin sharing that with her and also the bass player. In time the three of you could have a nice little library of music for two or three, whether folk, sacred, classical or some other type. Because you all worked it out to share, you will enjoy playing it, I’m sure! I doubt you will find much published music for harp, bass, and violin but that is a very nice combination, if done well.